Particulates or particulate matter from a diesel engine is mainly constituted by carbonic soot and a soluble organic fraction (SOF) of high-boiling hydrocarbon and contains a trace of sulfate (misty sulfuric acid fraction). In order to suppress such kind of particulates from being discharged to atmosphere, it has been carried out as shown in FIG. 1 that a particulate filter 4 is incorporated in an exhaust pipe 3 through which exhaust gas 2 flows from a diesel engine 1.
The particulate filter 4 is accommodated in a casing 5. Arranged in the casing 5 on an entry side of the particulate filter 4 is a fore oxidation catalyst 6.
As shown in (a) of FIG. 2, the particulate filter 4 comprises a filter body 7 in the form of a porous honeycomb structure made of ceramics such as cordierite and having lattice-like compartmentalized passages 8. Alternate ones of the passages 8 in the filter body 7 are plugged at their inlets with plugs 9 and the remaining passages with unplugged open inlets are plugged at their outlets with the plugs 9. Thus, only the exhaust gas 2 passing through porous thin walls 10, which compartmentalize the passages 8, is discharged downstream, particulates being captured on inner surfaces of the thin walls 10.
The particulates having been entrained in the exhaust gas 2 and captured by and accumulated on the inner surfaces of the thin walls 10 require to be appropriately burned off so as to regenerate the particulate filter 4 before exhaust resistance increases considerably due to clogging. However, the exhaust gas from the engine 1 in a normal operating status rarely has a chance to reach a temperature level at which the particulates spontaneously ignite. Thus, it has been developed into practical use that, in combination with the fore oxidation catalyst 6 arranged on the entry side of the particulate filter 4 in the casing 5 as mentioned in the above, used as the particulate filter 4 is a catalytic regenerative particulate filter 4 with an oxidation catalyst integrally carried by the filter body 7, said oxidation catalyst comprising, for example, alumina which carries platinum and is added with an appropriate amount of rare-earth element such as cerium.
Such combined use of the fore oxidation catalyst 6 with the catalytic regenerative particulate filter 4 accelerates oxidation reaction of the captured particulates to lower the ignition temperature, so that the particulates can be burned off even at the exhaust gas temperature lower than ever before.
However, in the particulate filter 4, the exhaust gas 2 flowing through the respective passages 8 stagnates to be lowered in flow rate just before the plugs 9 in the outlets, so that liable to be gradually deposited especially in this area is ash 11 generated by in-cylinder combustion due to additives in the lubricant and sulfur content in the fuel. The ash 11 as combustion residue requires to be periodically washed out in a maintenance operation of the particulate filter 4 since the ash accumulated too much may bring about substantial increase in exhaust pressure and deterioration of fuel economy.
As a specific way of washing the particulate filter 4, it has been proposed to wash out the combustion residue by jetting hot water under high pressure to the particulate filter 4 which is detached from the exhaust pipe 3 (see, for example, Reference 1).
[Reference 1] JP 2004-239072A